Gate photocathode for controlled single and multiple electron beam emission

ABSTRACT

A photocathode having a gate electrode so that modulation of the resulting electron beam is accomplished independently of the laser beam. The photocathode includes a transparent substrate, a photoemitter, and an electrically separate gate electrode surrounding an emission region of the photoemitter. The electron beam emission from the emission region is modulated by voltages supplied to the gate electrode. In addition, the gate electrode may have multiple segments that are capable of shaping the electron beam in response to voltages supplied individually to each of the multiple segments.

BACKGROUND

[0001] 1. Field of the Invention

[0002] This invention relates to electron beam sources and, moreparticularly, to photocathodes for the generation of single or multipleelectron beams.

[0003] 2. Prior Art

[0004] High resolution electron beam sources are used in systems such asscanning electron microscopes, defect detection instruments, VLSItesting equipment, and electron beam (e-beam) lithography. In general,e-beam systems include an electron beam source and electron optics. Theelectrons are accelerated from the source and focused to define an imageat a target. These systems typically utilize a physically small electronsource having a high brightness.

[0005] Improvements in optical lithography techniques in recent yearshave enabled a considerable decrease in the linewidths of circuitelements in integrated circuits. Optical methods, however, will soonreach their resolution limits. Production of smaller line-width circuitelements (i.e., those less than about 0.1 μm) will require newtechniques such as X-ray or e-beam lithography.

[0006] In e-beam lithography, a controllable source of electrons isdesired. A photocathode used to produce an array of patterned e-beams isshown in FIG. 1. U.S. Pat. No. 5,684,360 to Baum et al., “ElectronSources Utilizing Negative Electron Affinity Photocathodes withUltra-Small Emission Areas,” herein incorporated by reference in itsentirety, describes a patterned photocathode system of this type.

[0007]FIG. 1 shows a photocathode array 100 with three photocathodes 110comprising a transparent substrate 101 and a photoemission layer 102.The photocathode is back-illuminated with light beams 103 which arefocused on photoemission layer 102 at irradiation region 105. As aresult of the back-illumination onto photoemission layer 102, electronbeams 104 are generated at an emission region 108 opposite eachirradiation region 105. Other systems have been designed where thephotoemitter is front-illuminated, i.e. the light beam is incident onthe same side of the photoemitter from which the electron beam isemitted.

[0008] Often, light beams 103 or electron beams 104 are masked. In FIG.1, light beams 103 are masked using mask 106 which allows light ontoirradiation spots 108 but prevents light from being incident on otherareas of photoemission layer 102. FIG. 1 also shows mask 107 whichallows electrons to exit photoemission layer 102 only at certain surfacespots corresponding to emission regions 105. A photocathode may alsohave a mask between transparent substrate 101 and photoemission layer102 to block light beam 103 so that it is only incident at irradiationspots 105. In general, photocathode 110 may include no masking layers ormay have one or more masking layers.

[0009] Each irradiation region 105 may be a single circular spotrepresenting a pixel of a larger shape, the larger shape being formed bythe conglomerate of a large number of photocathodes 110 in photocathodearray 100. In that case, irradiation region 105 may be as small as ispossible given the wavelength of the light beam incident on photocathode100. Typically, a grouping of pixel irradiation regions has dimensionsof 100-200 μm. Each pixel can have dimensions (i.e. diameter) as low as0.1 μm. Alternatively, irradiation spot 105 and emission region 108 canbe a larger shape. In either case, the image formed by emission region108 will be transferred to e-beam 104 so long as the entirety ofirradiation region 105 is illuminated by light beam 103.

[0010] Photoemission layer 102 is made from any material that emitselectrons when irradiated with light. These materials include metallicfilms (gold, aluminum, etc.) and, in the case of negative affinity (NEA)photocathodes, semiconductor materials (especially III-V compounds suchas gallium arsenide). Photoemission layers in negative electron affinityphotocathodes are discussed in Baum (U.S. Pat. No. 5,684,360).

[0011] When irradiated with photons having energy greater than the workfunction of the material, photoemission layer 102 emits electrons.Typically, photoemission layer 102 is grounded so that electrons arereplenished. Photoemission layer 102 may also be shaped at emissionregion 108 in order to provide better irradiation control of the beam ofelectrons emitted from emission region 108. Further control of thee-beam is provided in an evacuated column as shown in FIG. 2.

[0012] Light beams 103 usually originate at a laser but may alsooriginate at a lamp such as a UV lamp. The laser or lamp output istypically split into several beams in order to illuminate each of focalpoints 105. A set of parallel light beams 103 can be created using asingle laser and a beam splitter. The parallel light beams may alsooriginate at a single UV source. Alternatively, the entire photoemissionarray 100 may be illuminated if the light source has sufficientintensity.

[0013] Photons in light beam 103 have an energy of at least the workfunction of photoemission layer 102. The intensity of light beam 103relates to the number of electrons generated at focal point 105 and istherefore related to the number of electrons emitted from emissionregion 108. Photoemission layer 102 is thin enough and the energy of thephotons in light beam 103 is great enough that a significant number ofelectrons generated at irradiation region 103 migrate and are ultimatelyemitted from emission layer 108.

[0014] Transparent substrate 101 is transparent to the light beam andstructurally sound enough to support the photocathode device within anelectron beam column which may be a conventional column or amicrocolumn. Transparent substrate 101 may also be shaped at the surfacewhere light beams 103 are incident in order to provide focusing lensesfor light beams 103. Typically, transparent substrate 101 is a glassalthough other substrate materials such as sapphire or fused silica arealso used.

[0015] If mask 106 is present either on the surface of transparentsubstrate 101 or deposited between transparent substrate 101 andphotoemission layer 102, it is opaque to light beam 103. If mask 107 ispresent, it absorbs electrons thereby preventing their release fromemission region 108. Mask 107 may further provide an electrical groundfor photoemission layer 102 provided that mask 107 is conducting.

[0016] Photocathode 100 may be incorporated within a conventionalelectron beam column or a microcolumn. Information relating to theworkings of a microcolumn, in general, is given in the followingarticles and patents: “Experimental Evaluation of a 20×20 mm FootprintMicrocolumn,” by E. Kratschmer et al., Journal of Vacuum ScienceTechnology Bulletin 14(6), pp. 3792-96, November/December 1996;“Electron Beam Technology—SEM to Microcolumn,” by T. H. P. Chang et al.,Microelectronic Engineering 32, pp. 113-130, 1996; “Electron BeamMicrocolumn Technology And Applications,” by T. H. P. Chang et al.,Electron-Beam Sources and Charged-Particle Optics, SPIE Vol. 2522, pp.4-12, 1995; “Lens and Deflector Design for Microcolumns,” by M. G. R.Thomson and T. H. P. Chang, Journal of Vacuum Science TechnologyBulletin 13(6), pp. 2445-49, November/December 1995; “Miniature SchottkyElectron Source,” by H. S. Kim et al., Journal of Vacuum ScienceTechnology Bulletin 13(6), pp. 2468-72, November/December 1995; U.S.Pat. No. 5,122,663 to Chang et al.; and U.S. Pat. No. 5,155,412 to Changet al., all of which are incorporated herein by reference.

[0017]FIG. 2 shows a typical electron beam column 200 using photocathodearray 100 as an electron source. Column 200 is enclosed within anevacuated column chamber (not shown). Photocathode array 100 may becompletely closed within the evacuated column chamber or transparentsubstrate 101 may form a window to the vacuum chamber through whichlight beams 103 gain access from outside the vacuum chamber. Electronbeams 104 are emitted from emission region 108 into the evacuated columnchamber and carry an image of emission region 108. Electron beam 104 maybe further shaped by other components of column 200.

[0018] Electron beams 104 are accelerated between photocathode array 100and anode 201 by a voltage supplied between anode 201 and photoemissionlayer 102. The voltage between photocathode array 100 and anode 201,created by power supply 208 (housed outside of the vacuum chamber), istypically a few kilovolts to a few tens of kilovolts. The electron beamthen passes through electron lens 204 that focuses the electron beamonto limiting aperture 202. Limiting aperture 202 blocks thosecomponents of the electron beams that have a larger emission solid anglethan desired. Electron lens 205 refocuses the electron beam. Electroniclenses 204 and 205 focus and demagnify the image carried by the electronbeam onto target 207. Deflector 203 causes the electron beam tolaterally shift, allowing control over the location of the image carriedby the electron beam on a target 207.

[0019] In 0.1 μm lithography systems, the size of a circular pixelincident on target 207 is on the order of 0.05 μm. Therefore, the imageof emission area 108 needs to be reduced by roughly a factor of 2 to 10,depending on the size of emission region 108. Target 207 may be asemiconductor wafer or a mask blank.

[0020] Conventional variable shaped electron beam lithography columnsshape the electron beam by deflecting the electron beam across one ormore shaping apertures. The resulting image in the shaped electron beamis then transferred to target 207 with a large total linear columndemagnification. The requirement of large total linear demagnification(supplied by electron lenses 204 and 205) results in large columnlengths, increasing electron-electron interactions that ultimately limitthe electron current density of the column. The low electron currentdensity results in a low throughput when the column is used inlithography.

[0021] Another major drawback in using known e-beam systems include theinability to modulate the electron beam without modulating the lightsource itself, usually a laser. Modulating a laser typically involves alarge amount of control circuitry, requiring a large amount of space,and can be slow. In addition, in a patterned array of photocathodes,modulation of individual photocathodes in the array is extremelydifficult. Finally, better resolution is required of lithography systemsin order to meet future demands of semiconductor materials processing.

SUMMARY

[0022] According to the present invention, a photocathode has a gateelectrode that modulates and, in some embodiments, shapes the emissionof an electron beam.

[0023] A photocathode emits electrons upon irradiation by a photon beamif the photon energy is greater than the work function of thephotocathode. By masking the photocathode selectively with an opaquematerial, the emission is confined to pre-defined regions. Providing anelectrically isolated gate structure that encompasses an emission regionof the photocathode allows the intensity of the electron beam to bemodulated by application of a gate bias voltage to the gate structure.If the gate structure has multiple segments, the electron beam emittedfrom the photocathode can also be shaped.

[0024] In a photocathode according to the present invention, an emissionarea is surrounded by a gate electrode that is offset from an electronemitting surface by an insulator. The gate electrode can be electricallycontrolled in order to turn the electron beam on or off or to vary theintensity of the electron beam. The electron beam is modulated in theregion between the gate electrode and electron emitting surface ratherthan at a light beam source such as a laser, resulting in fasterswitching times and space savings in the electron beam system.

[0025] Embodiments of this invention can be utilized to form an array ofphotoemission sources each having a precisely controlled emitting regionand position. In embodiments where the gate structure of each of thephotoemission sources in the array includes a single gate electrode,each of the single gate electrodes in the array may be individuallycontrolled or controlled in groups. In embodiments where the gatestructure of each of the photoemission sources in the array includesmultiple gate electrodes, each of the multiple gate electrodes may beindividually controlled or controlled in groups. In yet otherembodiments, the array of photoemission sources may include acombination of photoemission sources having a single gate electrode andphotoemission sources having multiple gate electrodes where each gateelectrode is individually or group controlled.

[0026] In general, emission regions can be of any size or shape that arewithin the limits of microfabrication technology. Some embodiments ofthe invention include self-biasing circuitry utilizing photoemission asthe feed-back for stable emission intensity.

[0027] A photocathode includes a transparent substrate and aphotoemission layer. The transparent substrate is transparent to a lightsource. The light source generates an array of light beams which arefocused on an array of irradiation regions directly above the emittingareas on the photoemission layer. In one embodiment, the light source isa laser and the array of light beams results from the laser beam beingsplit into multiple light beams using a beam splitter. Alternatively,the light source may be a UV lamp.

[0028] In some embodiments, each emitting area on the photoemissionlayer is a single pixel, a larger shape being formed by the aggregate ofall of the pixels. Alternatively, the emitting area itself may representany shape that is to be transferred to a target.

[0029] In some embodiments, masks are formed on top of the substrate inorder to form the light beams into the desired images before the lightbeams are incident on the irradiation region. Other embodiments place amask on the emitting surface of the photoemission layer. Yet otherembodiments place a mask between the photoemission layer and thesubstrate in order to form the image in the light beam. In someembodiments, a back surface of the substrate, where the light beams areincident and opposite the photoemission layer, is shaped to providelenses. The lenses help to focus the light beams onto the irradiationregion.

[0030] According to the present invention, the emitting area issurrounded by an insulator. The emitting area itself is left uncoveredby the insulator. In some embodiments, a single conductor is mounted onthe insulator to form a gate electrode. In other embodiments, multipleelectrically independent conductors are mounted around the emitting areaon the surrounding insulator to form a gate electrode having multiplesegments. Each segment of the gate electrode is independently controlledin order to turn on and off a corresponding portion of the electron beamthat is initiated at the emitting area.

[0031] A photocathode according to the present invention is suitable foruse in an arrayed electron source for conventional electron beamcolumns. Other embodiments of the invention are suitable for use as aminiaturized arrayed electron source for electron beam microcolumns.Some embodiments are suitable for use as a single gated source forconventional electron beam columns and microcolumns.

[0032] Photocathode arrays having gate electrodes with multiple segmentsallow variable shaping at the electron source in an electron beamlithography column without using shaping apertures or shaping optics.Use of these embodiments results in shorter column length because of thereduced need for further beam shaping and demagnification. The shortercolumn length results in less electron-electron interactions andultimately a higher throughput in systems such as lithography systemsbecause of the higher intensity electron beams.

[0033] The invention and its various embodiments are further discussedalong with the following figures and the accompanying text.

BRIEF DESCRIPTION OF THE FIGURES

[0034]FIG. 1 shows a patterned photocathode according to the prior art.

[0035]FIG. 2 shows a conventional electron beam column using thephotocathode shown in FIG. 1.

[0036]FIGS. 3A and 3B show a photocathode according to the presentinvention.

[0037]FIG. 4 shows a portion of a photocathode array having twophotocathodes according to the present invention.

[0038]FIG. 5 shows a photocathode according to the present inventionhaving a gate electrode with multiple segments.

[0039]FIG. 6A shows a photocathode according to the present inventionhaving multiple independent segments in the gate electrode.

[0040]FIGS. 6B and 6C show sample patterned e-beams resulting fromselectively turning on the segments shown in the gate electrode of FIG.6A.

[0041]FIG. 7 depicts the process of forming a photocathode according tothe embodiment of the invention presented in FIG. 4.

[0042]FIG. 8 shows a photocathode array according to the presentinvention.

[0043]FIG. 9 shows a micro-column utilizing a photocathode according tothe present invention.

[0044]FIG. 10 shows a multiple segment gated photocathode used in anelectron beam column where the beam shaping is accomplished at thephotocathode.

[0045]FIG. 11 shows a conventional variable shaped beam electron beamcolumn having multiple shaping components.

[0046] In the figures, components having the same or similar functionsare identically labeled.

DETAILED DESCRIPTION

[0047]FIGS. 3A and 3B show in a side view an embodiment of aphotocathode 300 according to the present invention. (The conventionalassociated housing, electrical leads, etc. are not shown.) In FIG. 3A, aphotoemitter 302 is deposited on a transparent substrate 301.Transparent substrate 301 is usually glass, fused silica or sapphire,although other transparent materials having structural strengthsufficient for support can be used.

[0048] A light beam 303 is incident on transparent substrate 301, passesthrough transparent substrate 301, and is absorbed by photoemitter 302at irradiation region 308. Photoemitter 302 emits electrons fromemission area 305, located on the surface of photoemitter 302 oppositeof irradiation region 308, when light beam 303 is incident uponirradiation region 308.

[0049] Emission area 305 can, in general, be of any shape and any sizewhere gate electrode 307 determines the electric field. Some usefulshapes include a circle, a square, a rectangle, an octagon and ahexagon. Irradiation region 308 should at least cover emission area 305.

[0050] A gate insulator 306 is deposited on photoemitter 302 such thatemission area 305 is surrounded, but not covered, by gate insulator 306.Gate insulator 306 may be made from any electrically insulating materialand is preferably made from SiO₂. Gate electrode 307 is deposited on theside of gate insulator 306 away from emission region 305. Gate electrode307 can be made from any conducting material.

[0051] Photoemitter 302 can be made from any material that emitselectrons when illuminated. The most efficient photoemitting materialsinclude gold, aluminum, and carbide materials. In addition, many III-Vsemiconductors, such as GaAs, are suitable photoemitter materials.Preferably, photoemitter 302 is made from gold and has a thickness ofabout 100 Å.

[0052] Photoemitter 302 will have a work function that is determined bythe actual photoemitter material. The work function is the minimumenergy required to release an electron from the material. The photons inlight beam 303 must have an energy at least as great as the workfunction in order that photoemitter 302 will emit electrons.

[0053] Light beam 303 is absorbed by photoemitter 302 at, or nearly at,the surface of photoemitter 302 corresponding to irradiation region 308.At that point, electrons will have a kinetic energy equal to the photonenergy minus the work function. These electrons migrate from irradiationregion 308 to emission area 305 and are emitted from the material atemission area 305 provided that the electrons have not lost too muchenergy to collisions within the photoemitter material. As such, thethickness of photoemitter 302 should be sufficient to absorb light beam303 but not so thick as to reabsorb a significant number of the freeelectrons created.

[0054] It is also desirable that, in embodiments of this invention, thekinetic energies of the emitted electrons not be too great, preferablyless than 0.5 eV but can be as great as a few eV, so that the emittedelectrons can be reflected by a voltage applied to gate electrode 307.If photoemitter 302 is gold, then a light beam having a photonwavelength of 257 nm or less is needed to produce photons having anappropriate photon energy.

[0055] Transparent substrate 301 must be transparent to light beam 303so that the maximum amount of light possible is incident on irradiationregion 308. Transparent substrate 301 can be of any thickness butpreferably is a few millimeters thick. In addition, light beam 303 maybe focused to cover irradiation spot 308 in an area corresponding toemission region 305.

[0056] The intensity distribution of light beam 303 is generallygaussian in shape, therefore light beam 303 will be more intense at itscenter than at its edges. Light beam 303 is preferably focused in such away that its intensity is nearly uniform across irradiation region 308so that electron beam 304 has nearly uniform intensity. In general,however, light beam 303 can be as focused as is desired.

[0057] Gate electrodes 307 are mounted to insulators 306 and can beconstructed from any conducting material. The thickness of gateinsulator 306 is preferably about 1000 Å and the thickness of gateelectrode 307 is also preferably about 1000 Å. In one embodiment,photoemitter 302 is held at ground voltage and gate electrode 307 isbiased at a voltage greater than ground, approximately +10 V, in orderto accelerate the electrons that are emitted from photoemitter 302. Gateelectrode 307 is biased at voltages less than ground, approximately−10V, in order to reflect the emitted electrons back towardsphotoemitter 302. Moreover, stable emission can be achieved by couplinga resistor 311 between photoemitter 302 and gate electrode 307 and usingthe emission-intensity for feed-back (i.e., a self-biasing system). Forexample, when electron emission increases the gate voltage decreasescorrespondingly which in turn lowers the emission intensity.

[0058] Anode electrode 310 is held at a voltage of from a few kilovoltsto several tens of kilovolts and accelerates the electrons out ofphotocathode 300 and into an evacuated electron beam column.Alternatively, photoemitter 302 is held at a high negative voltage, gateelectrodes 307 are biased at ±10 V compared to photoemitter 302, andanode electrode 310 is grounded.

[0059] In FIG. 3A, gate electrode 307 is held at +10 V. This voltage ischosen so as to be consistent with the electric field which would be setup between anode electrode 310 and photoemitter 302 if insulator 306 andgate electrode 307 were absent. With the voltage of gate electrode at 10V, electron beam 304, which carries the image of emission region 305, isaccelerated out of emission region 305. Insulators 306 and gateelectrode 307 also act as a mask in order to better shape the image ofemission region 305 contained in electron beam 304.

[0060] In FIG. 3B, gate electrode 307 is held at −10V. At this voltage,the electrons emitted by emission region 305 are accelerated backtowards emission region 305 by the electric field created between gateelectrode 307 and photoemitter 302. No electron beam 304 is createdbecause the electrons emitted from emission region 305 are reflectedback into photoemitter 302 rather than being accelerated away fromphotoemitter 302. Instead of an electron beam, electron cloud 309 iscreated where electrons are emitted out of photoemitter 302 and promptlyaccelerated back into photoemitter 302.

[0061] In some embodiments of the invention, the voltage at gateelectrode 307 is varied in order to control the intensity of theelectron beam. The higher the voltage difference between gate electrode307 and photoemitter 302 the greater the number of electrons that leavephotocathode 300. The maximum number of electrons available, those thatare emitted from emission region 305 as a result of light beam 303, areextracted when the gate electrodes are set at full on (about 10V).

[0062] Although the examples shown here have the gate biasing voltage at+10V for full-on operation and −10V for full-off operation, otherparameters for gate voltages are possible. The full-on bias voltage andthe voltage applied to anode electrode 310 determines the thickness ofinsulator 306 because the electric field created by gate electrode 307at the full-on bias voltage should be consistent with that field whichwould exist in the absence of gate electrode 307 and gate insulator 306.The full-off bias voltage limits the incident light beam photon energybecause in full-off operation the electrons emitted from emission region308 must be reflected back into photoemitter 302. In addition, gateelectrode 307 should be the dominant feature determining the electricfields near emission region 308. The size of emission region 308 istherefore limited by the relative sizes and distances between gateelectrodes 306 and anode electrode 310.

[0063] In embodiments where switching times are important, the RC timeconstant of gate electrode 307 should be relatively small. The spacingbetween gate electrodes, the spacing between the gate electrode and thephotoemitter, and the thickness of the electrodes determine the RC timeconstant and therefore the maximum rate of switching.

[0064]FIG. 4 shows an embodiment of a photocathode array 400. In FIG. 4,two emission regions 402 of photocathode array 400 are shown and bothemission regions 402 are illuminated by light beam 403 whichsimultaneously illuminates the entire portion of photocathode array 400shown. Parallel light beams 403 could be used instead with each beambeing focused on an individual emission region 402. Photocathode array400 comprises a transparent substrate 401, a conductor 408, gateinsulator 406, gate electrodes 407, and photoemitters 402.

[0065] Conductor 408, which can be made from any conducting material butis preferably aluminum, is deposited on transparent substrate 401 andhas an opening within which photoemitter 402 is mounted. Photoemitter402 may be any material which emits electrons when illuminated withphotons, as was previously discussed. Again, photoemitter 402 has a workfunction and the photons in light beam 403 must have an energy at leastas great as the work function in order that electrons are emitted fromemission region 405. Transparent substrate 401 is preferably glass butcan be any material that is transparent to light beam 403 such assapphire or fused silica. Conductor 408 is opaque to light beam 403 anddoes not emit electrons from its front when illuminated from the back bylight beam 403. Conductor 408, therefore, acts as a mask and defines anirradiation region 408. Emission region 405 lies directly oppositeirradiation region 408 on photoemitter 402 and can be of any shape andany size where gate electrodes 407 dominate the electric field.

[0066] A gate insulator 406 is mounted on conductor 408 and has anopening 410 such that photoemitters 402 are not covered by insulators406. Gate electrodes 407 are deposited on gate insulator 406. In thisembodiment, gate electrodes 407 overhang opening 410 by an amountsufficient to cause the electric fields created at emission area 405 tobe not substantially distorted by gate insulator 406. As in FIGS. 3A and3B, gate electrode 407 has the ability to turn electron beam 404 on andoff with a voltage applied to gate electrode 407. The on and off voltageroughly correspond to +10V and −10V, respectively. In addition, ultimateelectron beam intensity may be regulated by varying the gate electrodevoltage. A self-biasing resistor 411 also may be connected between gateelectrode 407 and conductor 408 in order to provide feedback forcontrolling the intensity of electron beam 404 by self-biasing.

[0067] In the photocathodes shown in FIGS. 3A, 3B and 4, the intensityof the electron beams may be controlled by controlling the actualvoltage between the gate electrode and the photoemitter. The lower thevoltage, the less intensity that the electron beam will have becausefewer of the electrons will escape the electron cloud where theelectrons have a statistical distribution of velocities in the directionof electron beam propagation. In addition, the gate electrodes may beused to regulate the intensity of the resulting electron beam. In someembodiments, a resistor is placed between the gate electrode and thephotoemitter so that a self-biasing feedback is created, i.e., ifemission increases, the gate voltage lowers correspondingly.

[0068] In FIG. 4, gate electrode 407 is shown as being the same for eachemission area 405. However, in general each emission area 405 has a gateelectrode 407 that is electrically isolated from the other gateelectrodes. In addition, a gate electrode for a particular emission areamay include several segments each of which are electrically isolatedfrom all of the others.

[0069] In some embodiments, the gate electrode surrounding the emissionregion has multiple segments. Multiple segments allow the ability toturn on parts of the emission region while turning off other parts ofthe emission region, shaping the image carried by electron beam 404.

[0070]FIG. 5 shows a photocathode as in FIG. 3 but with a right gatesegment 510 and a left gate segment 511 instead of single segment gateelectrode 307. The result of this construction is that the electron beamcan be selectively switched on. For example, in FIG. 5 right gatesegment 510 is held full-on at a bias voltage of 10 V and left gatesegment 511 is held full-off at a bias voltage of−10 V. The resultingelectric field reflects electrons which are emitted by emission region305 near left gate segment 511 while accelerating electrons are emittedout from emission region 305 near to right gate segment 510 ofphotocathode 500. The resulting electron beam 504 is an image of, inthis example, half the emission region 305. The resulting electron beam504 distribution is not uniform and is most intense near right gatesegment 510 and is essentially off at a point midway between the twosegments 510 and 511.

[0071]FIG. 6A shows in a plan view a four segment gate electrodeconfiguration. The gate segments are segment A 601, B 602, C 603, and D604. Emission region 305 in this example is a square. Emission region305 can be of any shape but is preferably a square. Other useful shapesinclude a circle, a rectangle, an octagon and a hexagon.

[0072]FIG. 6B shows in a plan view electron beam 504 that results whengate segments A 601, C 603, and D 604 are turned on (i.e., held at +10V)and gate segment B 602 is turned off (i.e., held at −10V). FIG. 6C showsin a plan view electron beam 504 that results when gate segments A 601and D 604 are turned on while gate segments B 602 and C 603 are turnedoff. Other shaped electron beams can be formed by selectivelycontrolling the voltages of the segments of the gate electrodes. Thisability lends great versatility to constructing photocathode arrays thatare useable for a variety of different tasks. FIG. 10 shows aphotocathode having a segmented gate electrode used in an electron beamcolumn for electron beam lithography.

[0073] In general, any number of gate segments can be used. The moregate segments there are, the more control a user of the photocathode hasover the electron beam created from a given emission area. This abilitymay be of great importance in efficiently writing features ontosemiconductor substrates. In addition, resistors can be coupled betweenindividual segments of the gate electrode and the photoemitter in orderto provide self-biasing control over electron beam intensity asdescribed above.

[0074] The photocathodes described above are conducive tominiaturization and precise integration into multiple photocathodesources. A photocathode array can be constructed on a single substratewith precise positioning of photocathodes. In particular, FIGS. 7A-7Fillustrate a process of manufacturing the photocathode illustrated inFIG. 4 using conventional semiconductor processing steps. Theillustrated process shows only a single photocathode of the photocathodearray. However, one skilled in the art can produce a photocathode arrayhaving precisely placed photocathodes with various emission area shapesand gate structures from this illustration. In addition, one skilled inthe art can modify this process in order to manufacture otherphotocathodes according to this invention or alter this process in waysthat result in the same photocathode construction.

[0075]FIG. 7A shows in a cross sectional view the first step in theprocess where an opaque layer of conducting film is deposited on atransparent substrate 401 such as glass, fused silica, or sapphire.Preferably, transparent substrate 401 is a glass substrate. As shown inFIG. 7B, the conducting film is masked and a window having anappropriate size and shape to form an emission area 410 is etchedthrough the conducting film. A gate insulator 406 is then deposited ontop of conducting film 408 and also fills the window of emission area410. Gate insulator 406 can be any electrical insulator but preferablyis SiO₂. A gate electrode layer 407 is then deposited on top of gateinsulator 406 as shown in FIG. 7D.

[0076] Gate insulator 406 is then masked and a hole 411 is etchedthrough gate electrode layer 407 and insulating film 406 as is shown inFIG. 7E. Hole 411 is aligned with emission area 410 and is slightlylarger than emission area 410. In addition, all of insulating film 406is removed from the window of emission area 410 by this etch.

[0077] In FIG. 7F, a selective isotropic etch has created a recessedhole 412 in insulating film 406 so that gate electrode 407 now overhangsthe opening created at hole 411 and recessed hole 412. Finally,photocathode material 402 is deposited using a directional depositiontechnique such as thermal evaporation from a point source or ionizedsputter deposition. This final deposition forms a photocathode 400 witha self-aligned gate aperture and is formed such that the photocathode iselectrically connected to conducting layer 408 but maintains electricalisolation from gate electrodes 407.

[0078] In addition, in an array of photocathodes manufactured by thisprocess, each gate electrode segment surrounding each of thephotocathodes may be formed by appropriately masking the gate insulator406 during deposition of gate electrode layer 407. Alternatively, gateelectrode layer 407 may be individually etched to form individual gatesegments. Also, interconnect lines that connected gate electrodesegments to pads can be formed along with the gate electrode segments ormay be deposited at a later process step.

[0079] As an alternative manufacturing method, the substrate could becoated with conducting layer 408, gate insulator 406 and gate electrode407 first. Window 411 is then etched through all films down totransparent substrate 401. Using a selective isotropic etch, the openingin gate electrode 407 could be enlarged slightly with respect to thecorresponding window 410 in conducting layer 408. Also alternatively,multiple segments of gate electrodes are created around each of holes411 by isotropically etching insulating breaks in gate electrode 407.

[0080] In some embodiments, the surface of substrate 401 may be shapedin order to focus the light beam onto an irradiation regioncorresponding to emission area 410 of photoemitter 402. Also, in someembodiments, photoemitter 402 may itself be shaped so as to better focusthe resulting electron beam that is emitted from the photocathode.

[0081]FIG. 8 shows in a plan view a four by four array of patternedphotocathodes. Emission areas 801 in this example are squares althoughany shape, including circles, rectangles, hexagons and octagons, can befabricated. Gate electrode 804 fully surrounds each emission area 801.Although only a single segment gate electrode is shown in FIG. 8, gateelectrode 804 may in general be constructed of multiple electrodesegments for further control of the electron emission from emission area801. Gate electrode 804 is connected to a bonding pad 803 by aninterconnect line 802. Both bonding pad 803 and interconnect 802 arepreferably made from the same material as is gate electrode 804 but anyconductor making electrical contact with gate electrode 804 can be used.In general, for lithography systems it is desirable that the physicalseparation between two adjacent emission regions be such that the arrayis a square. The minimum separation between emission regions isapproximately four times the physical dimensions of the emission region.In FIG. 8, the dimension of the square emission region with currentmicrofabrication technology can be as small as 0.1 μm. Preferably, theside dimension of the emission region is 0.1 μm. Therefore, the wholefour by four array shown in FIG. 8 is constructable within a square 1.6μm on a side, which is well within conventional microfabrication limits.

[0082]FIG. 9 shows in a side view a photocathode array 910 according tothis invention mounted within a microcolumn 900. Microcolumn 900 iscontained within an evacuated chamber (not shown). The substrate ofphotocathode array 910 may suffice as a vacuum window allowing a laserlight source onto the irradiation regions of photocathode array 910 oralternatively photocathode array 910 may be fully enclosed in the vacuumchamber. Electron beams 911 are emitted from the emission regions ofphotocathode array 910 and, depending on the control inputs to gateelectrodes 909 of photocathode array 910, are accelerated through anode901. Anode 901 is held at a voltage of from one kilovolt to several tensof kilovolts over that of the photoemitters in photocathode 910.Limiting aperture 902 blocks a portion of beams 911 which have a largeremission solid angle than desired. Deflector 903 allows the image of theemission regions contained in electron beams 911 to be laterallyshifted. Einzel lens, having electrodes 904, 905, and 906, focuses anddemagnifies the image onto target 907. Target 907 may be either asemiconductor wafer or a mask blank for electron beam lithography.

[0083] Photocathode array 910 can include any number of individualphotocathodes. Each of the individual photocathodes can include a singlesegment gate or a multiple segment gate. The image formed in electronbeam 911 is dependent upon the emission areas of each of the individualphotocathodes and the states of the gate electrodes of each of theindividual photocathodes. For example, a photocathode array 910 havingone photocathode with a single segment gate can only produce an image ofthe emission area of the photocathode. With a photocathode array 910having multiple photocathodes, each with an individually controlledsingle segment gate, various images can be formed by selectively turningon the individually controlled photocathodes to form conglomerates ofthe images of each of the emission areas of the “on” photocathodes. Aphotocathode array 910 where some of the photocathodes havemultisegmented gate electrodes have the most versatility because imagescan be formed using portions of emission areas of the individualphotocathodes.

[0084]FIG. 10 shows an electron source 1001 having a single photocathode1004. Photocathode 1004 has an emission area 1002 and a four segmentgate structure 1003. The four segment gate structure is capable ofselectively imaging emission area 1002. In the example of FIG. 10, thefour segment gate structure 1003 is used to shape an electron beam imageequivalent to one half of emission area 1002. The electron beam carryingthe electron beam image is accelerated out of photocathode 1004 byextraction electrode 1005. Demagnification lens 1006 demagnifies theelectron beam image onto wafer or mask blank 1008 to form the finalshaped beam image. The system shown in FIG. 10, having a minimal numberof components, allows shaped electron beam columns to be constructedutilizing a minimum amount of space.

[0085]FIG. 11 shows a conventional variable shaped electron beam column,in contrast to the electron beam column shown in FIG. 10. An electronbeam is formed at electron source 1101. Electron source 1101 may be athermionic cathode such as lanthanum hexaboride, LaB₆, or a single gatedphotocathode similar to that shown in FIG. 3. The electron beam isshaped by square aperture 1102 to form a shaped electron beam. Theshaped electron beam is focused by electron lens 1103 into region 1110.Spot shaping deflector 1104 deflects the electron beam at focus region1110 so that the shaped electron beam is shifted. The shaped electronbeam is then passed through square aperture 1105 to form an intermediateshaped electron beam. Square aperture 1105 passes that portion of theelectron beam that overlaps with the aperture and blocks that portion ofthe electron beam outside the aperture so that only a portion of theimage formed by square aperture 1102 is passed into the intermediateshaped beam image. Demagnification lens 1106 demagnifies the image andfocuses the image onto a final shaped beam image 1108 on a wafer or maskblank 1109.

[0086] The above described examples are demonstrative only. Variationsthat are obvious to one skilled in the art fall within the scope of thisinvention. As such, this application is limited only by the followingclaims.

We claim:
 1. A photocathode for supplying an electron beam whenilluminated, comprising: a transparent substrate; a photoemitter on thetransparent substrate, the photoemitter having an irradiation region atthe transparent substrate and an emission region opposite theirradiation region; a gate insulator on the photoemitter and surroundingthe emission region; and a gate electrode on the gate insulator, whereinthe gate electrode comprises at least one segment, the at least onesegment being electrically isolated so that the electron beam ismodulated and shaped in response to a voltage that is individuallyapplied to the at least one segment.
 2. The photocathode of claim 1 ,further comprising at least one resistor connected between the at leastone segment of the gate electrode and the photoemitter thereby providinga self-biasing feedback.
 3. The photocathode of claim 1 , wherein thegate electrode includes one segment completely surrounding the emissionarea.
 4. The photocathode of claim 1 , wherein the gate electrodeincludes four segments distributed evenly around the emission region. 5.The photocathode of claim 1 , wherein the gate electrode includes eightsegments.
 6. The photocathode of claim 1 , wherein the emission regionis circular.
 7. The photocathode of claim 6 , wherein the emissionregion has a diameter of 0.1 μm to 10 μm.
 8. The photocathode of claim 1, wherein the emission region is a square.
 9. The photocathode of claim8 , wherein an edge of the square has length of 0.1 μm to 10 μm.
 10. Thephotocathode of claim 8 , wherein the gate electrode includes foursegments, each of the four segments being located along an edge of thesquare.
 11. The photocathode of claim 1 , wherein the emission region isselected from a group consisting of a rectangle, a hexagon, or anoctagon.
 12. The photocathode of claim 1 , further including an opaquemask on the transparent substrate opposite the photoemitter, the opaquemask shaping an incident beam of radiation, the incident beam ofradiation being incident on the irradiation region.
 13. The photocathodeof claim 1 , wherein the photoemitter is selected from one of the groupconsisting of gold, aluminum, and carbide.
 14. The photocathode of claim1 , wherein the transparent substrate is of glass, fused silica orsapphire.
 15. The photocathode of claim 1 , wherein the gate insulatoris of SiO₂.
 16. A photocathode for supplying an electron beam whenilluminated, comprising: a transparent substrate; a conductor on thetransparent substrate, the conductor defining a window; a photoemitterlocated so that an irradiation region of the photoemitter is in contactwith the transparent substrate through the window of the conductor, thephotoemitter having an emission region opposite the irradiation region;a gate insulator on the conductor and surrounding the photoemitter; anda gate electrode on the gate insulator, wherein the gate electrodecomprises at least one segment, the at least one segment beingelectrically isolated so that the electron beam is shaped and modulatedin response to a voltage that is individually applied to the at leastone segment.
 17. The photocathode of claim 16 , further comprising atleast one resistor connected between the at least one segment of thegate electrode and the photoemitter to provide self-biasing feedback.18. The photocathode of claim 16 , wherein the gate electrode includesone segment completely surrounding the emission area.
 19. Thephotocathode of claim 16 , wherein the gate electrode includes foursegments distributed evenly around the emission region.
 20. Thephotocathode of claim 16 , wherein the gate electrode includes eightsegments.
 21. The photocathode of claim 16 , wherein the emission regionis circular.
 22. The photocathode of claim 21 , wherein the emissionregion has a diameter of 0.1 μm to 10 μm.
 23. The photocathode of claim16 , wherein the emission region is a square.
 24. The photocathode ofclaim 23 , wherein an edge of the square has length of 0.1 μm to 10 μm.25. The photocathode of claim 23 , wherein the gate electrode includesfour segments, each of the four segments being deposited along an edgeof the square.
 26. The photocathode of claim 16 , wherein the emissionregion is selected from a group consisting of a rectangle, a hexagon, oran octagon.
 27. The photocathode of claim 16 , further including anopaque mask on the transparent substrate opposite the photoemitter, theopaque mask shaping a beam of incident radiation, the beam beingincident on the irradiation region.
 28. The photocathode of claim 16 ,wherein the photoemitter is selected from one of a group consisting ofgold, aluminum, and carbide.
 29. The photocathode of claim 16 , whereinthe transparent substrate is of glass, fused silica or sapphire.
 30. Thephotocathode of claim 16 , wherein the gate insulator is of SiO₂.
 31. Aphotocathode array for supplying a multiple electron beam source whenilluminated, comprising: at least one photocathode, the at least onephotocathode having a gate electrode that modulates the electron beam inresponse to a voltage applied to the gate electrode; and at least onepad, the at least one pad being connected to the gate electrode of theat least one photocathode by an interconnect line, the at least one padallowing external control of the gate electrode of the at least onephotocathode.
 32. The photocathode array of claim 31 , wherein the gateelectrode of the at least one photocathode includes at least onesegment.
 33. The photocathode array of claim 31 , wherein the gateelectrode of the at least one photocathode includes four segments. 34.The photocathode of claim 31 , wherein the gate electrode of the atleast one photocathode includes one segment.
 35. The photocathode ofclaim 31 , wherein the gate electrode of the at least one photocathodeincludes eight segments.
 36. A method of fabricating a photocathode,comprising: forming a conducting film on a transparent substrate;etching a window through the conducting film; forming an insulating filmon top of the conducting film; forming a gate electrode film on top ofthe insulting film; etching an opening through the gate electrode filmand the insulating layer, the opening being aligned with the windowthrough the conducting film; recessing the opening in the insulatinglayer; forming a photoemission layer into the opening in the gateelectrode film so that a photoemitter is inserted into the windowthrough the conducting film.
 37. The method of claim 36 , whereinrecessing the opening in the insulating layer includes isotropicallyetching the insulating layer.
 38. The method of claim 36 , whereinforming a photoemission layer includes directionally depositing materialusing thermal evaporation or ionized sputtering.
 39. The method of claim36 , wherein the photoemission layer is selected from one of a groupconsisting of gold, aluminum, and carbide.
 40. The method of claim 36 ,wherein the transparent substrate is glass, fused silica or sapphire.41. The method of claim 36 , wherein the insulating film is SiO₂. 42.The method of claim 36 , further including: shaping the transparentsubstrate to irradiation incident radiation onto the window.
 43. Themethod of claim 36 , further including shaping the photoemitter tofurther shape electron emission.
 44. The method of claim 36 , furtherincluding selectively etching the gate electrode film in order to form agate electrode having multiple segments.
 45. A method of fabricating aphotocathode, comprising: forming a conducting film on a transparentsubstrate; forming an insulating film on the conducting film; forming agate electrode film on the insulating film; etching an opening throughthe gate electrode film, the insulating film, and the conducting film,the opening in the conducting film forming a window; recessing theopening in the insulating layer; forming a photoemission layer into theopening in the gate electrode film so that a photoemitter is insertedinto the window through the conducting film.
 46. The method of claim 45, wherein recessing the opening in the insulating layer includesisotropically etching the insulating layer.
 47. The method of claim 45 ,wherein forming a photoemission layer includes directionally depositingmaterial using thermal evaporation or ionized sputtering.
 48. The methodof claim 45 , wherein the photoemission layer is selected from a groupconsisting of gold, aluminum, and carbide.
 49. The method of claim 45 ,wherein the transparent substrate is glass, fused silica or sapphire.50. The method of claim 45 , wherein the insulating film is SiO₂. 51.The method of claim 45 , further including: shaping the transparentsubstrate to irradiation incident radiation onto the window.
 52. Themethod of claim 45 , further including shaping the photoemitter tofurther shape electron emission.
 53. The method of claim 45 , furtherincluding selectively etching the gate electrode film in order to form agate electrode having multiple segments.